The use of focused ion-beam (FIB) microscopes has become common for the preparation of specimens for later analysis in the transmission electron microscope (TEM). The structural artifacts, and even some structural layers, in the device region and interconnect stack of current integrated-circuit devices can be too small to be reliably detected with the secondary electron imaging in a Scanning Electron Microscope (SEM), or FIB, which offers a bulk surface imaging resolution of approximately 3 nm. In comparison, TEM inspection offers much finer image resolution (<0.1 nm), but requires electron-transparent (<100 nm thick) sections of the sample mounted on 3 mm diameter grid disks.
Techniques were later developed for cutting out and removing specimens for examination that required little or no preliminary mechanical preparation of the initial semiconductor die sample before preparation in the FIB. These lift-out techniques include an “ex-situ” method that is performed outside the FIB chamber, and “in-situ” methods performed inside the FIB.
The in-situ lift-out technique is a series of FIB milling and sample-translation steps used to produce a site-specific specimen for later observation in a TEM or other analytical instrument. During in-situ lift-out, a sample of material (usually wedge-shaped) containing the region of interest is first completely excised from the bulk sample, such as a semiconductor wafer or die, using ion-beam milling in the FIB. This sample is typically 10×5×5 μm in size. Removal of the lift-out sample is then performed using an internal nano-manipulator in conjunction with the ion-beam assisted chemical vapor deposition (CVD) process available with the FIB tool. A suitable nano-manipulator system is the Omniprobe AutoProbe 200, manufactured by Omniprobe, Inc., of Dallas, Tex. The material deposited in the CVD process is typically a metal or an oxide.
A TEM sample holder is then positioned in the field of view in the FIB, and the nano-manipulator is used to lower the lift-out sample onto the edge of the sample holder. The CVD metal deposition capability inside the FIB vacuum chamber is then used to secure the sample to the TEM sample holder. Once the sample is attached to the TEM sample holder, the probe-tip point is separated from the sample by ion milling. The portion of the method involving the operations that include the TEM sample holder is referred to as the “holder attach” step. The sample may then be milled using conventional FIB milling steps to prepare a thin area for TEM inspection or other analysis. Details on methods of in-situ lift-out may be found in the specifications of U.S. Pat. Nos. 6,420,722 and 6,570,170. These patent specifications are incorporated into this application by reference, but are not admitted to be prior art with respect to the present application by their mention in this background section.
The in-situ lift-out technique has become popular, because this method allows one to both take advantage of the FIB's unique capabilities and to extend these capabilities to the inspection of structures and defects in next-generation devices. Due to the small ion-beam spot size achievable by new FIB instruments (e.g., <10 nm), FIB specimen preparation techniques presently offer the best spatial resolution where site specificity is required.
A variation of this in-situ lift-out process involves “backside milling” of the lift-out sample. This variation was developed in response to the problem known as the “shower curtain” effect, in which non-uniform high-density materials on the surface of an integrated circuit produce a non-planar face on the lift-out sample after final thinning for TEM preparation. These non-planar surfaces have vertical ridges, parallel to the ion beam direction, that result from slower ion milling rates of denser materials near the top of the sample, where the top is defined as the edge closest to the ion beam source. Such non-uniform layers are quite common in integrated circuits, such as copper or aluminum interconnect traces and tungsten contacts. A planar surface on the lift-out sample in the area thinned for TEM inspection is important for TEM techniques such as electron holography. Backside milling involves inverting the sample before final thinning, so that high density materials in or near the active layer of the integrated circuit no longer impact the result of ion milling.
The process of in-situ lift-out can be simplified into three successive steps. The first is the excision of the sample using focused ion-beam milling and extraction of the sample from its trench. The second is the “holder-attach” step, during which the sample is translated on the probe-tip point to the TEM sample holder. Then it is attached to the TEM sample holder (typically with ion beam-induced metal deposition) and later detached from the probe-tip point. The third and final step is the thinning of the sample into an electron-transparent thin section using focused ion beam milling.
A significant portion of the total time involved in completing a TEM sample with in-situ lift-out is spent during the holder-attach step. The relative amount of time involved depends on the amount of time required to mechanically isolate the lift-out sample from the initial bulk sample (ion beam milling rate), but will vary between 30% to 60% of the total time for TEM sample preparation. The elimination of the holder-attach step provides several key revenue and resource-related advantages, due to the elimination of the need to transfer and attach the TEM sample to the TEM sample holder.
For example, without the holder-attach step the semiconductor wafer could be returned to a process flow immediately after lift-out. Thinning of the sample can be performed later in an off-line FIB. This reduces the load on the critical in-line (clean room) FIB, which makes lift-out for process control more practical, and reduces the level of expertise required by process engineers who operate the in-line FIB.
In order to eliminate the holder-attach step, the probe-tip point with the sample attached can be directly joined to the material that will form the TEM sample holder by a suitable method that will preserve the attachment between the lift-out sample and probe-tip point, and prevent the probe-tip point and the sample from separating from the sample holder during storage or inspection in the TEM. The assembly should not interfere with the normal operation of the TEM or other intended analytical instrument, and should survive well in the internal environment of the TEM, or other intended analytical instrument. These suitable methods include, but are not limited to, mechanical deformation of the sample holder material or probe-tip point material, or both; electrical or thermal bonding (such as, electrical welding) of the probe-tip point to the TEM sample holder material; bonding the probe-tip point to this material with a suitable glue or adhesive; bonding the probe-tip point to the TEM sample holder material with a CVD or evaporated material; or other suitable means. This direct attachment of the probe-tip point with the sample attached to it, to the TEM sample holder can be performed inside or outside the vacuum chamber of the FIB or other analytical instrument.